Generally golf balls are one of two types: solid or wound. One piece solid balls are typically made of polybutadiene, monomers, fillers and other materials. They are durable, easy to manufacture and are inexpensive. However, they do not provide ideal distance or spin, thus they are usually used as range or practice balls.
Two piece solid balls usually have a solid polymeric core and a cover. The core is typically made of a polybutadiene which is chemically cross-linked with zinc diacrylate and/or similar crosslinking agents. The core is then covered with a durable cover typically made with an elastomer or ionic copolymers such as SURLYN of E. I. DuPont de Nemours & Company. These balls are generally very durable and provide good distance because of their high initial velocity. However, they have a low spin rate due to their hardness and this results in their being difficult to control.
Wound balls are usually constructed with a solid or liquid filled center that is wound with yards of stretched elastic thread. A durable material such as SURLYN or a similar material, or a softer material such as balata or polyurethane is used to cover the wound core. Wound balls typically have better spin and feel characteristics than two piece balls. These balls are used by advanced players as they have more control over the ball's flight because the ball has a better spin and feel. However, these balls are structurally complex, and as such they are harder to manufacture. Wound golf balls are relatively more expensive than solid balls.
Many methods for manufacturing balls with liquid filled centers are disclosed in the prior art. For example, a liquid filled center can be made by vulcanizing two rubber hemispheres and applying an adhesive to the outer "lip" of the hemispheres and assembling the two hemispheres to create a sphere and subsequently vulcanizing the sphere. Liquid is injected into the sphere through use of a hypodermic needle and the resulting puncture hole is sealed with a patch material such as urethane isocyanate. As described in U.S. Pat. No. 4,443,322, the hemispheres can be submerged in a desired liquid before the two halves are joined and either vulcanizing the sphere while submerged or upon removal from the liquid.
Liquid-filled golf balls have certain fluid dynamic properties. The golf ball center fluid dynamics affect initial ball spin rate and rate of spin decay. These properties affect golf ball flight.
At impact between the club head and the ball, the tangential force transferred from the clubhead to the rigid structure of the ball, including the cover, windings and shell, will cause the rigid structure to spin. However, the inertial effects of the fluid mass will cause the spin of the fluid to lag behind that of the rigid structure of the ball. The force required to set the fluid in rotational motion is the frictional drag. Rotational kinetic energy from the spinning ball is transferred to the fluid by the drag force. This transfer of kinetic energy causes the spin rate of the rigid structure of the ball to drop or decay. Spin decay continues until the fluid and the ball are spinning at the same rotational speed.
The initial ball spin rate and the rate of spin decay affect the flight of the golf ball. Thus, if the fluid dynamic properties are altered to affect the initial spin rate and decay of the ball, the flight path of the ball can be altered. Therefore, golf ball performance can be improved by altering the fluid dynamic properties inside the shell of the golf ball.
The center shells of golf balls are designed and manufactured so that the inner surface is intentionally smooth. The only method presently available to modify the fluid dynamics within the center of the ball is to adjust the physical properties of the fluid within the center shell. Thus, the ability to alter the fluid dynamic properties within the center of the golf ball is limited by selection of a fluid. Accordingly, a ball is needed that will allow a golf ball designer to have greater control over the center fluid dynamics of a ball. Further, a ball is needed where fluid dynamic properties can be altered to affect initial spin rate and spin decay which will in turn affect overall ball performance by alteration of its flight path.